Fix mypy errors:
src/_pytest/runner.py:36: error: "addoption" of "OptionGroup" does not return a value [func-returns-value]
src/_pytest/helpconfig.py:64: error: "addoption" of "OptionGroup" does not return a value [func-returns-value]
src/_pytest/terminal.py:67: error: "_addoption" of "OptionGroup" does not return a value [func-returns-value]
src/_pytest/terminal.py:75: error: "_addoption" of "OptionGroup" does not return a value [func-returns-value]
If `PYTEST_DISABLE_PLUGIN_AUTOLOAD` is set, disable auto-loading of
plugins through setuptools entrypoints. Only plugins that have been
explicitly specified are loaded.
ref #3784.